Call them "born again" undecideds: Republicans exploring bids for the presidency in 2012 have ramped up their religious fervor and sharpened answers to questions about faith in an effort to court social conservative voters in key early primary states.

"I believe in God. I am Christian. I think the Bible is certainly, it is 'the' book," real estate mogul Donald Trump told the Christian Broadcasting Network last week after catapulting to second place in a poll of unofficial GOP presidential contenders.

But as voters begin to scrutinize the lives of a wide-open field of unofficial GOP presidential contenders, several personal histories might raise red flags in some religious circles.

Former Minnesota Gov. Tim Pawlenty's transition from the Catholic Church to evangelical Protestantism in the 1990s after marrying his wife, Mary -- a move he explains in his book "Courage to Stand" as an effort to "merge my faith and my church life" -- could hurt his appeal among some Catholic primary voters, several Catholic political activists said.

"To the degree that Catholics know about Pawlenty's conversion, they won't like it," ABC News political contributor Cokie Roberts said.

Meanwhile, former House Speaker Newt Gingrich might have curried favor among Catholics with his high-profile conversion to the church two years ago, leaving his Baptist roots behind. Some observers believe the shift, which came as he also sought public forgiveness for his marital infidelity, could also help him among Christians in general by demonstrating that he has been spiritually reborn.

"Newt's conversion could affect his candidacy in an indirect way by helping him explain some of his decisions," Brian Burch, president of CatholicVote.org, said of Gingrich's three marriages and confessed affair with a hill staffer, who later became his wife.

Trump has also been put on the spot by Christian evangelicals for his two failed marriages. "I'm a very hard worker, and I've always said it's very difficult for a woman to be married to me because I work all the time," he told CBN's David Brody when asked to explain why they failed.

And then there's former Massachusetts Gov. Mitt Romney's Mormonism, which stunted his bid in 2008, and could remain a touchy subject with religious conservative voters in 2012.

"Mormonism for most conservative Christians who have read about Mormonism, who have investigated it, would be in a different category," said Chuck Hurley, president of the Iowa Family Policy Center, who has begun sizing up the potential nominees making swings through the state. "I have met people who call themselves Mormon that I think are pretty biblically orthodox, but the deeper you look into the roots and tents of Mormonism, the more divergence you find."

Religious leaders in Iowa and South Carolina, where evangelicals wield significant influence in caucuses and primaries, praised the leading likely candidates for their testimonies of faith and orthodox positions on issues such abortion and same-sex marriage.

But when it comes down to picking a nominee, they say, they're really looking for genuine religiosity, which may be a problem for some Republican hopefuls who've stumbled along the road to Damascus.

"We will not just go by what they say and what we see," Hurley said. "It's much more important what they really, really believe. Do they walk the talk."